In recent years, devices compliant to formats such as MPEG (Moving Picture Experts Group) or the like have come into widespread use in information distribution such as broadcasting and information reception in general households. The devices handle image information as digital signals, take advantage of redundancy peculiar to the image information in order to perform highly effective information transmission and accumulation at that time, and compresses the image by orthogonal transform such as discrete cosine transform or the like and motion compensation.
In particular, MPEG2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is defined as a general-purpose image encoding format, and is a standard encompassing both of interlaced scanning images and sequential-scanning images, and standard resolution images and high definition images. For example, MPEG2 has been widely employed now in broad range of applications for professional usage and for consumer usage. By employing the MPEG2 compression format, a code amount (bit rate) of 4 to 8 Mbps is allocated to a standard resolution interlaced scanning image having 720×480 pixels, and 18 to 22 Mbps is allocated to a high resolution interlaced scanning image having 1920×1088 pixels, for example, whereby a high compression rate and excellent image quality can be realized.
MPEG2 is mainly intended for high image quality encoding adapted to broadcasting usage, and a lower code amount (bit rate) than the code amount of MPEG1, i.e., an encoding format having a higher compression rate is not handled. With the widespread of mobile terminals, needs for such an encoding format will be increased from now on, and in response to such needs, standardization of the MPEG4 encoding format has been performed. With regard to an image encoding format, the specification was confirmed as international standard as SOC/IEC 14496-2 in December in 1998.
Further, in recent years, standardization of a standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Experts Group)) has progressed, originally intended for image encoding for videoconferencing usage. Compared with the conventional encoding techniques such as MPEG2 and MPEG4, H.26L requires a larger amount of calculation in encoding and decoding, but is known to achieve higher encoding efficiency. Also, currently, as a part of the activity of MPEG4, standardization for taking advantage of functions not supported by H.26L to realize higher encoding efficiency has been performed based on the H.26L as Joint Model of Enhanced-Compression Video Coding.
As a schedule of standardization, H.264 and MPEG-4 Part10 (AVC (Advanced Video Coding)) become an international standard in March, 2003.
Further, as an extension thereof, FRExt (Fidelity Range Extension) including a coding tool necessary for business use such as RGB, 4:2:2, or 4:4:4, 8×8 DCT (Discrete Cosine Transform) and quantization matrix stipulated by MPEG-2 has been standardized in February, 2005, whereby AVC can be used as an encoding format capable of suitably expressing even film noise included in movies, and has come to be employed for wide ranging applications such as Blu-Ray Disc (registered trademark) and the like.
However, nowadays, needs for higher-compression encoding have been increased, intending to compress an image having around 4000×2000 pixels, which is quadruple of a high-vision image, or to distribute a high-vision image within an environment with a limited transmission capacity like the Internet. Therefore, in VCEG (Video Coding Expert Group) under the control of ITU-T, studies related to improvement of encoding efficiency have been continuously performed.
By the way, the macroblock size of 16×16 pixels is not the best for large image frames such as UHD (Ultra High Definition: 4000×2000 pixels) that will be handled in next-generation encoding methods.
Currently, with a view to further enhancing the encoding efficiency compared than AVC, standardization of an encoding format called HEVC (High Efficiency Video Coding) has been in progress by JCTVC (Joint Collaboration Team—Video Coding) that is a joint standardization body of ITU-T and ISO/IEC (for example, see Non-Patent Document 1).
IN the HEVC encoding format, a coding unit (CU) is defined as a processing unit similarly to the macroblock in AVC. The CU is not fixed to 16×16 pixels like the macroblock in AVC, and is designated in each sequence in the image compression information.
By the way, in MPEG2 and MPEG4, for example, in a sequence like a fade scene, where a motion exists and brightness is changed, an encoding tool for absorbing the change of the brightness is not prepared. Thus, there is a problem of decreasing the encoding efficiency.
To solve the problem, a weighted prediction process is provided in AVC (for example, see Non-Patent Document 2). In AVC, whether the weighted prediction is used can be designated in units of slices.
Further, in AVC, the weighted prediction can be applied in units of slices. Furthermore, a method of applying the weighted prediction in units of blocks (intensity compensation) is also disclosed (for example, see Non-Patent Document 3).